I own a Bronica ETRS system and I've been looking around for a few more film backs. Simple question: can I use 120 film in a 220 back? Will the counter just be off? Or will it simply get to the end of the roll and not advance further without continually firing the shutter?

i don't know about that system, but the backs i have for my mamiya have a spool that moves in and out to accommodate 120 and 220 rolls. if you use the 220 film with 120 setting i believe it will rewind prematurely.

does the back you have accommodate both? or can you at least find out somehow?

220 has a different style paper backing that 120, this is to keep the spacing between frames the same, with 120 film in a 220 back the spacing will widen as the frame count goes up, the opposite occurs with 220 film in a 120 back at the end of the roll frames will over lap

I have used 120 in my 220 back for my ETRSI on more than one occasion including weddings and the only thing that was off was the frame numbers. Of course I would not figure this out for the first time on something important but it has worked for me.

I wouldn't try it for anything mission critical. The 220 back will be pressing extra hard on your film since it expects that there's no paper backing. Sure it will keep the film flat, but you could jam. I've even had 120 jam in a 120 back because the paper was too thick (Bergger).